If he has any intelligence, he'll conclude, as you do, that his own merit is suspect. However, you have not convinced him by merely telling him so. Instead of pointing out his own inadequacy, such as it is, why not use examples to demonstrate that he is either right or wrong, but leave him, the person, out of it.

Sorry, no matter how you slice it, when you attempt to make the argument about the person offering an argument, you raise the dreaded (and highly popular, judging by all the responses below) ad hominem. Unfortunately for their authors, they have been listed among the informal fallacies (relevance), for well over 2000 years now. When they are tendered, they're every bit as irrelevant (and wrong) today as they were when Socrates and Plato were explaining them.

Let me illustrate why this is so:

"All youz guys smell like dog excrement."

"So do you, you're covered in it!"

"So, does that make me correct or incorrect...do you, or do you not, smell like dog excrement?"

"Uh...yeah, I guess we do."

QED

Had they replied, "Yeah, and it stinks, but you might want to include yourself in that category," he might have replied, "Nope, I am not included." In that case, you could prove that he is wrong. It would still be an aside to his original statement, but at least you would have to bear the burden of proof. When you sidestep his original argument, you have not proved him wrong, only that you feel he also suffers from the same defect, an incidental and largely irrelevant opinion.

BTW, I teach this stuff at the university level, so I'm pretty comfortable with it.

"Well I hope you are less dull and pretentious when you are teaching this stuff at uni"

Sorry but my knowledge of Latin is very rusty so I was wondering if my example is an ad hominem or a tu quoque or maybe both?